1. Field of the Invention
The present invention relates to a semiconductor device having a heterojunction. The present invention is useful particularly for a bipolar transistor having an SiGeC layer formed by epitaxial growth.
2. Description of the Related Art
A heterojunction bipolar transistor (HBT) made up of single-crystal SiGeC and single-crystal Si is disclosed in JP-A No. 68479/2001. As FIG. 24 hereof shows in section, this HBT consists of a collector, a base, and an emitter. The collector comprises a layer 101 of n-type single-crystal Si and a layer 102 of n-type single-crystal SiGeC. The base is a layer 103 of p-type single-crystal SiGeC. The emitter comprises a layer 104 of n-type single-crystal SiGeC and a layer 105 of n-type single-crystal Si.
The conventional HBT has the profiles of compositional ratios of Ge and C and concentration of Boron (B) which are distributed as shown in FIG. 25 hereof. It is to be noted that the compositional ratios of Ge and C increase in the collector in going from layer 101 (layer of n-type single-crystal Si) to layer 102 (layer of n-type single-crystal SiGeC), gradually decrease in the base in going from layer 102 to layer 104, and further decreases in the emitter in going from layer 104 (layer of n-type single-crystal SiGeC) to layer 105 (layer of n-type single-crystal Si).
The HBT constructed as shown in FIG. 25 has an energy band gap structure (the lower end of the conduction band and the upper end of the valence band) as shown in FIG. 26. FIG. 26A shows the band structure for a small injection of electrons from the emitter, and FIG. 26B shows the band structure for a large injection of electrons from the emitter. It is to be noted that the energy of the conduction band in the base 103 (layer of p-type single-crystal SiGeC) decreases in going from the emitter to the collector according as the compositional ratios of Ge and C change.
In the conduction band, there is no energy barrier due to band gap at the interface between layer 101 (layer of n-type single-crystal Si) and layer 102 (layer of n-type single-crystal SiGeC layer) forming the collector. Consequently, electrons that are injected from the emitter travel through the base with an acceleration due to an electric field created by the inclined conduction band. See FIG. 26A.
An example of HBT having a heterojunction of single-crystal SiGe and single-crystal SiC is disclosed in JP-A No. 77425/2000. As FIG. 27 shows in section, this HBT is constructed of a collector (consisting of a layer 106 of n-type single-crystal Si and a layer 107 of n-type single-crystal SiC), a base (which is a layer 108 of p-type single-crystal SiGe), and an emitter (consisting of a layer 109 of n-type single-crystal SiC and a layer 110 of n-type single-crystal Si).
It would be desirable to have an HBT which has a heterojunction between a layer of single-crystal Si, a layer of single-crystal SiGe, and a layer of single-crystal SiGeC, capable of high-speed operation even with a large injection of electrons from the emitter.
The present invention is intended to address the following problems encountered in the conventional technologies.
In the conventional bipolar transistor having the base formed from single-crystal SiGeC, the collector consists of a layer 101 of single-crystal Si and a layer 102 (FIG. 24) of n-type single-crystal SiGeC placed directly thereon which has a smaller bandgap than single-crystal Si. This structure induces the following phenomenon. As electrons injected from the emitter increase, electrons from the base diffuse into the depletion layer adjacent to the collector at the base-collector junction, thereby canceling out space charges due to n-type impurity ions. This substantially expands the neutral base. The consequence is that an energy barrier appears in the conduction band at the depletion layer of the base-collector junction. This energy barrier impedes the diffusion of electrons injected from the emitter, which in turn deteriorates the HBT""s high-speed performance. FIG. 26 is an example of the energy bandgap of the HBT constructed as shown in FIG. 24. FIG. 26A is a band structure for a small injection of electrons from the emitter, and FIG. 26B is a band structure for a large injection of electrons from the emitter. It is clearly understood that the neutral base expands as the injection of electrons from the emitter increases.
One possible way to prevent the operating speed of HBT from being decreased by the energy barrier despite a large injection of electrons from the emitter is to make thicker the layer 102 (FIG. 24) of n-type single-crystal SiGeC. However, this is not practical because improving the crystallinity requires decreasing the growth temperature. Since the growth rate of the SiGeC layer exponentially decreases in inverse proportion to the growth temperature, a thick layer of single-crystal SiGeC takes a very long time to grow. This leads to a low throughput and a high cost in the manufacture of SiGeC HBT.
Another example of the conventional HBT is shown in FIG. 27. It has a base layer 108 of single-crystal SiGe and a layer 106 in the collector of single-crystal Si, with a layer 107 of n-type single-crystal SiC interposed between them. In this HBT, the collector has a larger bandgap than the base regardless of the injection of electrons from the emitter. This creates an energy barrier in the conduction band at the base-collector junction. This in turn poses a problem of impeding the diffusion of electrons. As a result, high-speed performance deteriorates.
The essential features of the present invention are summarized below with reference to FIGS. 1 and 6. FIG. 1 is a sectional view showing the laminate construction of the main region of the HBT of the present invention. FIG. 6A is a schematic diagram showing the energy band structure of a preferred HBT of the present invention in the normal operating state. FIG. 6B is a schematic diagram showing the energy band structure of a preferred HBT of the present invention which manifests itself when the neutral base extends to the collector. These figures show the lower end of the conduction band and the upper end of the valence band. The references of numerals are as follows: 16: emitter, 9: base, 7 and 3: collector (3 denoting the region of the semiconductor substrate)
The present invention provides an HBT having the base and collector layers such that no energy barrier appears in the conduction band at the depletion layer of the base-collector junction. In addition, no energy step occurs in the HBT of the present invention in the neutral base when the injection of electrons from the emitter is large.
The HBT of the present invention is achieved by forming the base and collector from single-crystal SiGe as the main material for the HBT in which single-crystal SiGeC exists at the heterojunction. Basically, the selection of the material is made such that the energy gap (Eg) of the base is larger than the Eg of the collector when the injection of electrons from the emitter is large. The above-mentioned single-crystal SiGeC may be used for the base and collector regions. Single-crystal SiGeC is preferably selected for the base or collector.
In the HBT of the present invention, the base preferably comprises a layer of single-crystal SiGe or single-crystal SiGeC, and the collector preferably comprises an SiGe layer or an SiGeC layer or a laminate of SiGeC layer and SiGe layer (which will be denoted by SiGeC/SiGe). The following table summarizes the preferred selections of the materials.
The emitter preferably comprises a single-crystal Si layer, a single-crystal SiGe layer, a laminate of single-crystal SiC layer and single-crystal SiGe layer (denoted by SiGe/SiC hereinafter), or a laminate of single-crystal SiGeC layer and single-crystal SiGe layer (denoted by SiGe/SiGeC hereinafter).
The present invention may be variously modified as follows within the scope thereof, although it complies with the conventional HBT technology for the thickness of each region.
The advantage of making the base from single-crystal SiGe is high-speed operation and improved current gain. Another advantage is the impossibility of degradation of crystallinity on account of impurities such as B which would be introduced along with C if the base were made from SiGeC.
However, when the base made from single-crystal SiGe is heavily doped with B (as an impurity), the base width expands as it diffuses into other regions. To remedy this drawback, it is desirable to make the collector from a layer of single-crystal SiGeC. C not only prevents the diffusion of B but also reduces the strain of the SiGeC layer because it has a smaller lattice constant than Si and Ge. The resultant collector is less likely to generate dislocations and defects resulting from a high-temperature annealing in the fabrication process of HBTs. This in turn suppresses leakage current. The advantage of making the collector from SiGeC or SiGeC/SiGe is that no energy barrier is formed in the depletion layer (adjacent to the collector) at the base-collector junction. This contributes to high-speed operation and to an improved current gain.
The advantage of introducing C into the base or making the base from single-crystal SiGeC is the prevention of diffusion of B (or an impurity introduced into the base). The suppression of impurity diffusion prevents the base width from expanding. This effect is particularly remarkable when the collector is made from SiGeC/SiGe. The result is that no energy barrier occurs in the conduction band at the depletion layer of the base-collector junction. This in turn leads to high-speed operation even though the injection of electrons from the emitter is large. In addition, the advantage of making the base from SiGeC and the collector from SiGeC/SiGe is the reduced strain in the SiGeC layer. The resultant base is less likely to generate dislocations and defects during annealing. This helps increase yields and reduce fluctuation in characteristic properties of HBTs. The C-free region in the collector permits a SiGe layer to selectively grow as the lower region of the collector in the semiconductor substrate. This structure is favorable to the production of HBTs.
The present invention can provide an HBT capable of high-speed operation even in the case where the injection of electrons from the emitter is large in an HBT which utilizes a heterojunction which is formed by using a single-crystal Si layer, a single-crystal SiGe layer, and a single-crystal SiGeC layer.
Moreover, according to another aspect of the present invention, it is possible to provide an HBT having a low production cost and a method for production thereof.